The invention relates to a method as well as a device for using mass-produced light-emitting diodes (LEDs) of a predetermined luminance, preferably in ophthalmic testing equipment, for example as stimuli in hemispherical perimeters.
It is known for example from German Pat. No. 2,507,723, to use LEDs as stimuli in perimetric ophthalmic testing equipment.
A prerequisite for precise test results is a uniform luminance of the LEDs located on the hemispherical viewing area of the perimeter. Heretofore, LEDs with approximately the same luminance were selected from a plurality of LEDs. This method is uneconomical, and allows certain variations in the luminance of the selected LEDs. Likewise, similar, but lesser, requirements for uniform luminance occur in equipment where LED readouts are utilized in place of meters and guages, such as VU meters in stereo cassette decks, receivers, and the like.
Thus, an object of the present invention is to permit the use of mass-produced LEDs at a precisely predetermined luminance, as required for stimuli in hemispherical perimeters, with practically zero rejects.
The above noted object is achieved according to an apparatus aspect of the invention by an LED arrangement wherein the LED is longitudinally adjustably retained in an elastic plastic tube which is sealed at one end by a light diffusing disc, and according to a method aspect of the invention by the use of conventional LEDs in the noted arrangement to meet precision requirements by adjusting the distance between the LED and the diffusing disc until the precise required luminance is achieved.
An important feature of the displaceable arrangement is that a conventional mass-produced LED can be mounted in a sleeve that is closed at the end from which the light emerges by a diffusing disc without care being taken to select a LED of a particular luminance. To achieve a uniform luminance at the end of the sleeve from which the light emerges, the LED is displaced in the sleeve by a precise measuring device lengthwise until the luminance at the end of the sleeve from which the light emerges corresponds exactly to a predetermined value. The precise measuring device can be, for example, a calibrated V(L) photocell that measures the luminance of the LED.
In this fashion, conventional LEDs with luminance values set with relatively coarse tolerance can be converted into components with very similar luminance. The elements can then be installed for example in hemispherical perimeters, whereby subsequent replacement of individual components poses no difficulties as far as variations in luminance are concerned.
In order to make the LEDs displaceable against friction in the sleeves, while holding them securely, the sleeves can be provided with projections or beads running lengthwise, said projections or beads being dimensioned so that they are elastically deformed by the LED.
The diffusing disc provided at the end of the sleeve from which the light emerges can be connected directly with the sleeve, but is can also be mounted with the aid of a cap which fits over the outside of the sleeve. The cap may be glued or screwed to the sleeve. In addition, the cap can be provided externally with mounting elements to hold the assembly at the point where it is to be installed. The end of the cap can be designed to suit the specific application. For example, for opthalmic testing equipment the cap may be made to closely match the shape and color of the viewing surface of a hemispherical perimeter, so that nonluminous stimuli will be scarcely perceptible to the viewer.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, a single embodiment in accordance with the present invention.